Fiber-optic data transmission generally imposes new requirements on clock synchronization. It is desirable to accomplish clock synchronization by optical means and dispense with optical-to-electrical transducers.
The invention starts from a synchronizing unit as is described, for example, in PCT Application WO 91/15907. This PCT application describes a transmission system which comprises a transmitting unit and a plurality of access nodes and in which the clocks of the access nodes are to be synchronized with the clock of the transmitting unit. The transmitting unit is connected to the access nodes by an optical fiber and transmits a bit sequence as a synchronization signal to the access nodes. In the access nodes, this synchronization signal is coupled from the fiber and fed to a synchronizing unit.
This prior art synchronizing unit contains an electrically controllable optical switching element, an optical-to-electrical transducer, a clock generator, a sequence generator, an integrator, and a controller.
The sequence generator generates a bit sequence at the clock rate of the clock generator. By means of the switching element, this bit sequence is correlated with the bit sequence extracted from the fiber, which serves as the synchronization signal. The transducer converts the resulting optical signal to an electric signal, which is integrated in the integrator, and the result of the integration is fed to the controller. By means of this result, the controller determines the offset between the clock generator and the clock signal on the fiber, and it controls the clock generator so that this clock difference disappears, i.e., that the clock generator is in lock with the clock signal on the fiber.
The disadvantage of this synchronization method is that it only represents a specific solution for a distributed switching system. It only permits clock synchronization for the transmission of data packets from a clocking station to one or more clock-synchronous stations. Thus, the method is not suitable for the transmission of data packets from terminals to a central exchange at the clock rate of the exchange, for example.